History, Fame, & Fortune
Bacteriophage P22's original claim to fame, in 1952, was that it was the first phage shown to be able to perform generalized transduction (1,2) (definition). It is now known that during assembly of progeny particles host DNA is "incorrectly" recognized for packaging by the P22 gene 3 protein about 2% of the time (3); each of the transducing particles thus formed contains one contiguous fragment of host DNA about 43.5 kbp long, which can be injected into a susceptible bacterium just as the phage chromosome is from normal virions. P22 can thus be used in the laboratory to move genes from one bacterium to another, and so is of practical importance in the study of Salmonella typhimurium genetics (2). It seems likely that this process is also active in horizontal transfer of genes among bacteria, especially among closely related phyla. Although work with phage lambda was hard on its heels, Peter Lobban used P22 as the first phage DNA cloning vector (4).

Morphology & classification
P22 is a temperate dsDNA phage, and it is a "lambdoid" phage in that it carries control of gene expression regions and early operons similar to those of phage lambda. However, the genes which encode the proteins that build the virion are quite different from those of lambda. The icosahedral (T=7; 5, 6) virion head is about 60 nm in diameter, and it has a very short tail. This virion morphology puts it in the formal Podoviridae group.

Relatives
Relatives with similar genomic transcription patterns and life cycles include lambda and all the other lambdoid phages; relatives with similar short-tailed virion morphology and DNA homology in the virion protein genes include phages L, epsilon34 and others. Many Podoviridae, for example phages T7 and phi29, have very little if any DNA similarity with P22, even though their virion morphologies are similar.

Hosts & cultivation of this phage
The exact history of the original isolation of P22 is unfortunately lost in mists of time, but we do know it was originally isolated after induction in about 1952 of a Salmonella typhimurium lysogen that was from either Sweden or Chile. It is only known to infect "smooth" strains of Salmonella typhimurium, i.e., those which have the O-antigen polysaccharide on their surface. To begin an infection, P22 virions bind to the O-antigen. The virion's tail fiber protein (a.k.a., baseplate, tailspike or gene 9 protein) has endorhamnosidase activity which cleaves the O-antigen chain (7). It may be that this binding and cleavage of O-antigen allows the virion to burrow its way to the surface of the bacteria's outer membrane (although such "burrowing" by P22 has not been demonstrated directly).

Genomics

§ P22 has a linear dsDNA chromosome within its virion that is about 43500 bp in length and has blunt ends, and it has a circular genetic map. However its "wild type" nucleotide sequence is 41725 bp (see below). The complete genome sequence is available online (accession number AF217253). The study of P22 has naturally focused on those aspects of its life cycle that are different from lambda. These areas are in part the mechanism by which it circularizes its DNA upon infection and the way it packages its DNA into virions.

§ The virion chromosome is packaged from a concatemer of the sequence that results from rolling circle DNA replication. It carries a direct duplication of about 4% of the DNA sequence at its two ends because the inside of the phage head has more space than is filled by 100% of the sequence (8). This is called "headful packaging" in that DNA from the concatemer is "stuffed" into the head until it is full (9,10), rather than putting exactly one copy of the sequence in the virion.

§ Linear P22 virion DNA must be circularized upon injection by a homologous recombination event between the direct repeats at the two ends of the chromosome. The machinery that does this can be the host rec gene products, but P22 also carries recombination function genes that act in the absence of the host enzymes to accomplish circularization (11). It is this circle, which contains exactly one copy of the P22 nucleotide sequence, that is the substrate for gene expression and DNA replication.

Assembly pathway
Like other large dsDNA viruses, P22 builds a protein "procapsid" (5, 12, 13) structure first and then places the DNA chromosome within this preformed container. A notable and well-studied feature of P22 procapsid assembly is that a "scaffolding protein" is required for construction of the procapsid. About 250 molecules of this scaffolding protein are present in the procapsid, but at about the time DNA enters the structure, all of the scaffolding protein leaves (13, 14). This released scaffolding protein is not damaged, and it can re-assemble with newly synthesized coat protein to make more procapsids. In a laboratory infection an average scaffolding protein molecule participates in about five rounds of procapsid assembly. It thus acts catalytically and was one of the first proteins that was found to mediate the assembly of other proteins (the phage coat protein in this case) without becoming part of the finished structure. The action of a scaffolding protein in procapsid assembly has turned out to be quite general in the assembly of large icosahedral virus capsids (including the herpesviruses of eukaryotes), but in some cases the scaffold is proteolytically removed instead of being recycled. In addition, the P22 scaffolding protein represses its own synthesis when not assembled into procapsids; this is one of the only examples known of a phage virion structural protein whose synthesis is modulated by the assembly process (15).

References
These are mostly recent references to give the reader access to up-to-date literature on the subject. Original references can be found within them.
1. Zinder, N., and Lederberg, J. (1952) Genetic exchange in Salmonella. J. Bacteriol. 64: 679-699.
2. Masters, M. (1996). Generalized transduction. in Escherichia coli and Salmonella typhimurium (Neidhardt, F., Ed.-in-chief), ASM press, Washington, D.C., pp2412-2441
3. Ebel-Tsipsis, J., Botstein, D., and Fox, M. (1972) Generalized transduction by phage P22 in Salmonella typhimurium. J. Mol. Biol. 71: 433-448.
4. Lobban, P., and Kaiser, A. D. (1973). Enzymatic end-to-end joining of DNA molecules. J. Mol. Biol. 78: 453-471.
5. Casjens, S. (1979). Molecular organization of the bacteriophage P22 coat protein shell. J. Mol. Biol. 131:1-14.
6. Zhang, Z., Greene, B., Thuman-Commike, P., Jakana, J., Prevelige, P. Jr., King, J., and Chiu, W. (2000). Visualization of the maturation transition in bacteriophage P22 by electron cryomicroscopy. J. Mol. Biol. 297:615-26.
7. Steinbacher, S., Baxa, U., Miller, S., Weintraub, A., Seckler, R., and Huber, R. (1996). Crystal structure of phage P22 tailspike protein complexed with Salmonella sp. O-antigen receptors. Proc. Natl. Acad. Sci. USA 93:10584-8.
8. Casjens, S., and Hayden, M. (1988). Analysis in vivo of the bacteriophage P22 headful nuclease. J. Mol. Biol., 189:467-474.
9. Streisinger, G. Edgar, R., and Stahl, M. (1967). Chromosome structure in phage T4. Circularity of the linkage map. Proc. Natl. Acad. Sci., USA 57: 775-779.
10. Casjens, S., Wyckoff, E., Hayden, M., Sampson, L., Eppler, K., Randall, S., Moreno, E., and Serwer, P. (1992). The bacteriophage P22 portal protein is part of the gauge that determines the length and packing density of intravirion DNA. J. Mol. Biol. 224: 1055-1074.
11. Susskind, M., and Botstein, D. (1978) Molecular genetics of bacteriophage P22. Microbiol. Rev. 42: 385-413.
12. King J., Lenk E., Botstein D. (1973). Mechanism of head assembly and DNA encapsulation in Salmonella phage P22. II. Morphogenetic pathway. J. Mol Biol. 80:697-731.
13. King, J., and Casjens, S. (1974). Catalytic head assembling protein in virus morphogenesis. Nature 251:112-119.
14. S. Casjens and R. Hendrix, (1988) "Control mechanisms in dsDNA bacteriophage assembly", in The Bacteriophages, volume 1, ed. R. Calendar, Plenum Press, p. 15-91.
15. Wyckoff, E., and Casjens, S. (1985). Autoregulation of the bacteriophage P22 scaffolding protein gene. J. Virol. 53:192-197.